Bi-directional energizing circuit for a stepping motor with means to prevent conduction in one coil unit previously energized coil conduction is extinguished

ABSTRACT

A stepping motor having its stator windings connected into two phases is advanced one step by reversing the direction of current flow in one of the phases. Each phase is connected to a D.C. source by a pair of paths with each path directing current in a different direction through the phase. The paths are alternately shiftable from conducting to non-conducting with the shift of a path to being conducting being delayed until current flow from the path shifted to non-conducting is essentially zero to prevent short circuiting between the paths.

United States Patent 1191 May Apr. 17, 1973 3,529,220 9/1970 Kobayashiet a]. ..3 1 8/ 1 3s [54] E Q g f g R 3,581,182 5/1971 Comstock..3l8/68S CIRCUIT 0 A T PPN O O 3,453.514 7/1969 Rakesetal. ..3l8/254XWITH MEANS TO PREVENT CONDUCTION IN ONE COIL UNIT PREVIOUSLY ENERGIZEDCOIL CONDUCTION IS EXTINGUISHED Inventor: Joe Cyril May, Cheshire, Conn.

Assignee: The Superior Electric Company,

Bristol, Conn.

Nov. 2, 1970 Filed:

Appl. No.:

US. Cl ..3l8/696, 318/439 Int. Cl. ..I-I02k 29/00 Field of Search.;..;3l8/254, 138, 696,

References Cited UNITED STATES PATENTS 6/1971 Kobayashi et al. ..3l8/254PULSE STEP CON ROL- Primary Examiner-G. R. Simmons Attorney-Johnson &Kline [57] ABSTRACT is essentially zero to prevent short circuitingbetween the paths.

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i9 I .mvsmozz BI-DIRECTIONAL ENERGIZING CIRCUIT FOR A STEPPING MOTORWITH MEANS TO PREVENT CONDUCTION IN ONE COIL UNIT PREVIOUSLY ENERGIZEDCOIL CONDUCTION IS EXTINGUISHED In U.S. Pat. No. 3,077,555 assigned tothe assignee of the present invention there is disclosed a controlcircuit consisting of a plurality of mechanical switches for changingthe energization of the stator windings of a stepping motor. In U.S.Pat. No. 3,280,395, also assigned to the assignee of the presentinvention, there is disclosed a motor control circuit using electroniccomponents for also changing the energization of the stator windings ofa stepping motor. Stepping motors that are capable of being controlledby both circuits are essentially identical with each having a pluralityof poles, with there being a winding for each pole and with the windingsbeing connected into two phases. The only difference between the motorsis that in the former instance there is a unitary winding, i.e., asingle coil on eachpole, and it is completely energized, while in thelatter instance each pole winding consists of two separate coils withonly one coil being energized at a time. The latter motor is sometimesreferred to as a bifilar motor and the magnetic polarity of a pole isset by which coil of the winding is energized, with one coil causing anN polarity and the other coil an S polarity.

While both circuits have been found to operate satisfactorily, the useof mechanical switches limits the frequency at which the changes ofenergization can occur, and hence the speed of the motor. Also thebifilar motor when operated by the motor control circuit only is capableof using one-half of the available windings, thus tending toreduce theavailable torque, at least over its low speed range, as compared tohaving both halves of the winding continually energized.

It is accordingly an object of the present invention to provide anenergizing circuit for a stepping motor hav' ing unitary windings whichuses electronic solid state components to change the direction ofcurrent in the 'stator windings in the motor, rather than mechanicalswitches, to enable faster operation of the motor.

Another object of the present invention is to provide an energizingcircuit for a stepping motor having bifilar windings in which bothportions of the windings are continually energized to thereby enable theavailable torque of the motor to be increased at least for the lowerstepping speeds without increasing the losses in the motor.

A further object of the present invention is to provide an energizingcircuit for a stepping motor which may be used with a motor havingeither unitary or bifilar windings.

Still another object of the present invention is to achieve the aboveobjects with a bi-directional energizing circuit which consists ofelectronic components which are relatively economical to manufacture,reliable in performance, durable in use and capable of being used withpresently existing stepping motors and operating circuits therefor.

The bi-directional energizing circuit of the present invention, whileusable in other and different applica-. tions, has particular utilitywhen used to control the energization of the windings of a steppingmotor. It is specifically herein shown as being connected between asubstantially constant current unidirectional power source, such asshown in U.S. Pat. No. 3,505,579 (assigned to the assignee of thepresent invention), and a stepping motor as disclosed in the twoabove-noted patents. The energizing circuit reacts to information fromthe pulse-to-step portion of the motor control circuit, such as shown inthe above-noted U.S. Pat. No. 3,280,395 to produce movement of thestepping motor in the same direction and at the same frequency ascommanded by the pulse-to-step portion. The stepping motor may be ofeither the unitary or bifllar winding construction, with the latterhaving both coils connected in parallel so that, when energized, all thewinding on a pole is energized. The stator windings are interconnectedto form two phases, with a change of direction of current in one of thephases producing an incremental movement of the motor and with thechanges being alternated between the phases.

The bi-directional energizing circuit includes a pair of conductingpaths connected to each phase with the paths being soconnectcd to thepower supply that when one of the pair is rendered conducting,unidirectional current flows in one direction through its associatedphase, while when the other of the pair of paths is rendered conducting,unidirectional current flows through the phase in the other direction.However, as each of the paths includes transistors of relatively highpower capacity and a signal, terminating conduction in a path does notinstantaneously open the path because there is a slight time delayduring which the transistor conducts because of inherent storage andfall time. Though the pulse-to-step circuit effectively directs thebi-directional energizing circuit to instantaneously shift theconductivity in the paths, if the paths also reacted instantaneously,then a short circuit would develop across the power supply to causemalfunctioning. However, the present invention incorporates into thebi-directional energizing circuit means for introducing a time delay tothe path being rendered conducting, withthe delay approximately equalingthe time which is required for the current in the path which is renderednon-conducting to essentially become zero, thereby preventing thedevelopment of a short circuit.

Other features and advantages will hereinafter appear.

In the drawings:

FIG. 1 is a block diagram of a stepping motor and associated circuitsincluding the bi-directional energizing circuit of the presentinvention.

FIG. 2 is an electrical schematic diagram showing the interconnectionsbetween portions of the bi-directional energizing circuit and thewinding phases of a stepping motor.

FIG. 3 is a representation of the value of current flow with respect totime in the pair of paths of one phase.

FIG. 4 is a block and logic diagram of the bidirectionalenergizingcircuit.

FIG. 5 is a block and schematic diagram of a base drive amplifier shownin block form in FIG. 4.

' Referring to FIG. 1, the bi-dire'ctionalenergizing circuit of thepresent invention is generally indicated by the reference numeral 10 andis used to control the power from a DC. power source 11 to a steppingmotor 12 under the direction of a pulse-to-step control 13. The DC.power source 11 is essentially a constant current source and is morefully disclosed in the abovenoted US. Pat. No. 3,505,579, with leads 14and 15 having unidirectional current therein.

The pulse-to-step control 13 is disclosed in US. Pat. No. 3,280,395 andit functions to accept pulses on a lead 16 and effects a change in-theenergization of its output leads A, A, B, and B for eaqi pulse. Thechanges are according to the sequence AB, A B, AB, AB, AB, etc., toadvance the motor a plurality of steps in one direction, while reversingthe sequence reverses the direction of movement of the motor.

The motor 12 is of the type disclosed in the abovenotecl two U.S.Patents and it is formed to have a first phase I and a second phase IIwith each phase consisting of a serial connection of the windings onalternate poles. If the motor is of the type which has each windingunitary, then only the winding 12a for phase I will be present; while,if the motor is of the type having bifilar windings, then winding 12aconsists of a series connection of one coil on each pole while the othercoils of the same poles are also serially connected together asindicated by the winding 12b and then connected in parallel with phase12a. It will be understood that the coils of each bifilar winding,instead of being serially connected together and then in parallel (as12a and 12b), may be individually connected in parallel at each pole andthen serially connected together.

The energizing circuit is connected to the windings by leads 17-20 withenergization of the leads 17 and 18 causing all windings in phase I tobe energized, while energization of leads 19 and 20 will cause allwindings in phase II to be energized. All four leads are maintainedenergized at all stepping positions of the motor so that all thewindings in the motor will thus be maintained energized.

In FIG. 2, the interconnection of the leads 17-20, phases I and II andthe power source leads l4 and are shown. The bi-directional energizingcircuit directs the current to phase I windings in one direction fromplus lead 15 to minus lead 1- 1, by a path which includesa transistor'a,lead 17 phase I windings, lead 18 and a transistor b. For conductingcurrent to phase I windings in the opposite direction, there is provideda path that includes a transistor 0, lead 17, phase I windings, lead 18and transistor d to the lead 14. Similarly, for the phase II windings,there is a first conducting path which includes transistors e and f,while the path for conducting current in the opposite direction throughthe phase II windings includes transistors g and h.

Diodes, such as diode 21, are connected as shown to provide a path forcurrents due to voltages induced in the windings by the change ofcurrent flow therein during the transfer interval. As a furtherprecaution against harm which high value induced voltages may cause,there is provided a peak clipper generally indicated by the referencenumeral 22, which is connected across the leads l4 and 15 and serves toshort circuit values of induced voltages above a predetermined maximumlevel. g

In operating the motor 12 to effect steps in response to pulses on thelead 16, the pulse-to-step control 13 provides, through thebi-directional energizing circuit, the above-noted sequence ofenergization on leads A through B. The phases are energized according tothe sequence I II, I ETITTII, I II, where a phase with a line thereoverindicates current direction through the phase opposite to currentdirection through the phase without a line thereover. For energizing thephases for the first sequence I II, when the control leads energizationis AB, transistors a and b for phase I and transistors e and f for phaseII are conducting. For the next step in the motor sequence, AB,transistors a and b are maintained conducting for phase I, whiletransistors g and h are rendered conducting and transistors e and fnon-conducting. For the next step, when control leads 5 are energized,only transistors c, d, g and h are conducting, while for the final stepof AB only transistors c, d, e and fare conducting. Accordingly, thebi-directional circuit 10 energizes the motor in the same sequence andat the same rate as commanded by the pulse-to-step control 13 toaccordingly produce the same number of steps and at the frequency whichthe pulses on the lead 16 command.

Referring to FIG. 3, there is shown, using time as a base and zerovoltages as a reference, the values of the current flow through thetransistors a and b and the transistors c and d. The first line 23indicates the value of current increasing to a level 23a relativelyrapidly when the transistors a and b are rendered conducting. Uponcausing the transistors a and b to be non-conducting at a point 24', thecurrent in the path that includes the transistors a and b will decreaseto zero as at 25. However, it will require a time interval to decreaseto zero, as indicated by the reference character X, and this is causedby the storage and fall time inherent in a transistor of relatively highpower capacity. Though the pulse-to-step control 13 provides directionsto simultaneously cease conduction of the path having transistors aand'b and initiate conduction in the path having transistors c and d,the bi-directional circuit of the present invention withholds therendering of conduction of transistors c and d for a period whichsubstantially equals the time X. Thus, though the transistors a and bwere directed to be non-conducting at-point 24, transistors c and d aremade conducting at a point 26, which essentially corresponds in timeto'the point 25. The current in the path having transistors c and d willthen build up to a level 27, which is numerically equal to the level23a.

Similarly, for the next change of energization of phase I, the commandfrom the pulse-to-step control 13 appears where indicated by thereference numeral 28, but the time for the current in the path oftransistors c and d to decrease to essentially zero is again indicatedby the reference character X. However, the beginning indicated by thereference numeral 29 of conduction of the path having transistors a andb is delayed for approximately the X time period. Accordingly, thebidirectional energizing circuit of the present invention not onlyprevents both paths to a phase from being simultaneously renderedconducting, but also assures that, before rendering one path conducting,the current flow in the path being rendered non-conducting hassubstantially decreased to zero.

Moreover, the prevention of simultaneous conduction is done in such amanner as will be herein understood as to enable the motor 12 to respondessentially instantaneously to the directions from the control 13. Thetime period X varies not only with different transistors but also withthe quantity of current being controlled thereby, with a typical periodbeing on the order of 50 microseconds for amperes for a germaniumtransistor.

Referring to FIG. 4, there is shown a block and logic diagram forcausing the transistors a-h to be rendered conducting andnon-conducting, as heretofore indicated. The pulse-to-step control 13 isindicated as having its leads A and A connected to a differentiator A,while the leads B and B are connected to a differentiator B indicated bythe reference numerals 30 and 31, respectively. The leads A and Acontrol the conduction in the windings of phase I and lead A is connected as an input to an AND gate 32, while the lead A is connected asan input to an AND gate 33. The output of the differentiator A 30 isapplied to a monostable multivibrator or one shot 34, whose output isapplied as the other inputs to the AND gates 32 and 33.

The differentiator A 30, upon receipt of a change of energization in theleads A and A, which are basically changes in voltage level on theleads, produces to the one shot 34 a voltage pulse rather than a voltagelevel. The one shot 34 upon receipt of a pulse shifts the voltage levelin its output from a 1 state to 0 state for a duration of time which isessentially equal to the time period X, mentioned in connection withFIG. 3, and thereafter the output voltage level of the one shot 34returns to its 1 state. Accordingly, when the voltage level from themotor control of the lead A is changed from 1 to O and the lead A from 0to l at the AND gate 32, there is an instantaneous 1 from the lead A anda l a time period X later from the one shot 34. The output of the ANDgate 32 is thus delayed by the time period X from going to l.

The AND gate 32 has an output which is one of the inputs to another ANDgate 35, which has as its other input unidirectional'power from a DC.source 350 so that it functions basically as a DC. amplifying switch. Akc clock 36 has essentially a square wave output at that frequency andconstitutes an input to a push-pull amplifier 37. The output of the ANDgate 35, with the input from the AND gate 32 being 1 by having both ofits inputs a l, and the clock 36 producing a square wave having voltagelevels of l and 0 will accordingly produce at the output of push-pullamplifier 37 an alternating square wave which shifts its direction ofconduction at a frequency corresponding to the frequency of the clock.

The square wave output of the push-pull amplifier is directed to a basedrive amplifier 38 and also to another base drive amplifier 39. Theamplifier 38 is connected to the base of transistor 0 and provides abase voltage which controls its conduction, while the base amplifier 39is connected to the base of transistor b and produces a voltage thereatwhich controls its conduction.

Referring to FIG. 5, the output of the push-pull amplifier 37 isconnected to a center-tapped primary winding 40 of a transformer havinga first centertapped secondary winding 41 and a second centertappedsecondary winding 42. A voltage in the winding 41 is rectified by therectifiers shown and, when applied to the base of a transistor 43,renders it conducting to supply a voltage to the base of transistor a toalso cause it to conduct. Similarly, the secondary winding 42 sup- 7plies a rectified unidirectional voltage to the base of a transistor 44which causes it to conduct and in turn apply a voltage at the base oftransistor b to cause it to conduct. It will be understood that as bothtransistors a and b must be rendered conducting simultaneously andconcurrently that, by having both of their base drive amplifiers 38 and39 connected to the same push-pull amplifier 37, such operation isassured. Also, though the output of the push-pull amplifier to theprimary winding consists of an alternating wave of relatively highfrequency that is rectified, there is basically supplied a constantvalue of unidirectional voltage at the base of the transistors 43 and 44because of the reaction time of the components and because it is asquare wave.

Referring again to FIG. 4, it will be understood that while transistorsa and b are maintained conducting, transistors c and d are caused to benon-conducting by reason of the lead A supplying to the AND gate 33 a 0voltage level signal which provides a 0 input to an AND gate 45. Eventhough the other input to the AND gate 45 is from the DC. power source350, the output of the AND gate 45 will continually be a O, which inturn prevents the producing of a square wave output by a push-pullamplifier 46 which is connected by base drive amplifiers 47 and 48 totransistors c and d. The transistors c and d are thus maintainednon-conducting.

The control circuitry for the transistors e, f, g and h is identical tothe circuitry for the transistors a-d and thus includes an AND gate 49having one input connected to the lead B and an output connected as aninput to an AND gate 50, which in turn is connected to a push-pullamplifier 51, the latter having its output connected to base driveamplifiers 52 and 53 for the transistors e and f. Similarly, the lead Bis connected to an AND gate 54 having its output connected as an inputto an AND gate 55, which in turn is connected to a push-pull amplifier56 whose output is connected to base drive amplifiers 57 and 58 fortransistors g and h. A one shot 59 similar to the one shot 34 isconnected to the differentiator B 31 and also has its output be an inputto the AND gates 49 and 54. The DC. power 35a is further connected toprovide inputs to the AND gates and while the clock 36 provides an inputto the push-pull amplifiers 51 and 56.

The various components in the logic circuit having the same functionhave the same construction and hence the circuit includes many duplicateparts. It is pointed out, however, that the power supply to the basedrive amplifiers is not identical in absolute values in view of therequirement that there be maintained a relative difference voltagebetween base and emitter of the transistors in the base driveamplifiers, such as the transistors 43 and 44. Thus, the power supply tothe base drive amplifiers for transistors b, c,fand g may be the same,while the power supply to the base drives and of the remainingtransistors is different. For example, in one application, the power tothe base drive amplifier for transistor a is 6 volts with respect to thevoltage in the lead 17 which, if assumed to be 0, is thus 6 volts inabsolute terms. Additionally, the transistor d is connected to the lead18, which is at 40 volts and requires that the base drive amplifier fortransistor b be about 34 volts. Such power supplies are generallyreferred to as floating power supplies and are used to provide constantrelative voltages between two points where one of the points may varywith respect to other points in the circuit.

the sequence of abgh, abef, cdef, ca'gh, abgh, etc. As

suming the first step of the sequence, the transistors a and b arerendered conducting by both inputs to the AND gate 32 being 1, and theinput to the AND gate 35 being 1, so that the inputs to the push-pullamplifier 32 are a steady DC and a shifting l and from the clock,thereby causing the push-pull amplifier 37 to effect operation of thebase drive amplifiers 38 and 39 of the transistors a and b. Similarly,the AND gate 49 has two 1 inputs, the AND gate 50 has 1 so thattheinputs to the push-pull amplifier 511 are a steady DC. and shifting 1and 0, with the result that the push-pull amplifier 51 produces a squarewave output to energize the base drive amplifiers 52 and 53 oftransistors e andf.

Upon the step-to-pulse control 13 shifting to the next sequence wherethe lead A is maintained energized, the lead l3 deenergized and the leadB energized, the elimination of the 1 input to the AND gate 49immediately ceases operation of the push-pull amplifier 51 to stopconduction of transistors e and f, though as shownin FIG. 3 there is aslight time delay in the decay of current flow through thesetransistors. The differentiator 31 applies to the one shot 59 a pulsesimultaneously with the change of energization of the leads B and E, theone shot provides a 0 level on the AND gate 54 for a period of timeapproximating the time for the current to decrease to essentially zeroin the other path and then reverts to its normal state to place a l atthe input of the AND gate 54. This causes the push-pull amplifier 56 tobecome operative, rendering transistors g and h conducting to effect thesecond step of the sequence,

For the third step, the lead B is maintained energized, while the lead Ais deenergized and the lead A energized. This places 1 on the AND gate33 and a O on the AND gate 32 causing the transistors a and b to berendered non-conducting. Shortly thereafter, at a time determined by theone shot 34, the AND gate 33 has two 1 inputs causing the push-pullamplifier 416 to become operative and rendering the transistors c and dconducting. It will be understood that the push-pull amplifiers, whenthere is no power from their associated AND gate, do not operate, as theinput DC. power thereto is absent.

It will thus be seen that the bi-directional energizing circuit changesthe energization of the motor windings at the same frequency andsequence as directed by its step-to-pulse control 13, and hence themotor operates as if the pulse-to-step control were controlling the motor. The time delay introduced by the one shots 34 and 59 merelyshortens, by a very small period, the length of time that the windingsare energized and does not alter the frequency or direction of thechange.

It has been found that the use of the bi-directional energizing circuitwith a bifilar motor renders the motor more efficient, especially atlower speeds, by energizing all the motor windings at the same time,because at low speeds (0 to 2000 steps per second) the copper losses inthe windings tend to predominate over iron losses in the stator poles.While it is desired to force as much current through the windings aspossible, the amount of heating and heat dissipation which occurs servesas an upper limit for the amount of current and hence limits the outputtorque of the motor. However, by having both bifilar windings in eachphase connected in parallel, it has been found that the resistance ofthe windings is decreased by half, which enables almost 40 percent morecurrent to be forced through the motor windings as compared to onlyenergizing one coil of each winding with the same temperature rise ofthe motors. This not only increases the output torque of the motorsubstantially, but also, as this type of motor has a tendency toresonate at very low speeds, i.e., 0 to steps per second, the highertorque in the lower speed range tends to damp out and minimize thistendency, thereby producing a more stable motor. Moreover, with a motorhaving only a unitary winding on each pole, the bi-directionalenergizing circuit of the present invention enables the motor to beoperated at a much faster speed than it was capable using heretoforeknown mechanical switching arrangements.

Variations and modifications may be made Within the scope of theinvention and portions of the improvements may be used without others.

I claim:

1. An energizing circuit for a stepping motor having a plurality ofwindings with all the windings being connected into either a first phaseor a second phase with a change in energization of one of the phasesproducing an incremental movement and in which each change consists of areversal of current flow through a phase comprising a source ofunidirectional current, pulse-tostep control means for providing signalsto produce a change of energization and a bi-directional energizingmeans interconnected between the source and the phases to controlcurrent flow through the phases from the source under the direction ofthe pulse-to-s'tep control means, said bi-directional energizing meanshaving for each phase a first path for directing current flow from thesource through the phase in one direction and a second path fordirecting current flow from the source through'the phase in the otherdirection, means for rendering each path conductive or nonconductive andmeans for preventing simultaneous operation of more than one pathincluding means for delaying after receipt ofa signal from thepulse-to-step control the initiation of a path from becoming conductiveand means for initiating terminating the conduction in one pathessentially simultaneously with the receipt ofa signal.

2. The invention as defined in claim 1 in which each path includes atransistor having an inherent time delay in ceasing conduction uponinitiation of termination so that current flows in one path afterinitiating termination of conduction for a short period and in which themeans for delaying provides a time that approximates the short period.

3. The invention as defined in claim 1 in which the pulse-to-stepcontrol means has a pair of leads for each phase with the signal for achange of energization being a change in voltage levels on' the leads,in which the means for rendering the paths conductive or non-conductiveincludes logic means in each path, there being one lead connected to onelogic means and in which the logic means is responsive to only onevoltage level on its lead to effect conduction and is responsive only tothe other voltage level to effect non-conduction.

5. The invention as defined in claim 1 in which the source has two leadswith a switching means connected between the source and the phase toprovide two switching means in each path and in which the means forrendering the paths conductive or non-conductive includes means forsimultaneously identically altering the state of conduction of bothswitching means.

1. An energizing circuit for a stepping motor having a plurality ofwindings with all the windings being connected into either a first phaseor a second phase with a change in energization of one of the phasesproducing an incremental movement and in which each change consists of areversal of current flow through a phase comprising a source ofunidirectional current, pulse-tostep control means for providing signalsto produce a change of energization and a bi-directional energizingmeans interconnected between the source and the phases to controlcurrent flow through the phases from the source under the direction ofthe pulse-tostep control means, said bi-directional energizing meanshaving for eaCh phase a first path for directing current flow from thesource through the phase in one direction and a second path fordirecting current flow from the source through the phase in the otherdirection, means for rendering each path conductive or nonconductive andmeans for preventing simultaneous operation of more than one pathincluding means for delaying after receipt of a signal from thepulse-to-step control the initiation of a path from becoming conductiveand means for initiating terminating the conduction in one pathessentially simultaneously with the receipt of a signal.
 2. Theinvention as defined in claim 1 in which each path includes a transistorhaving an inherent time delay in ceasing conduction upon initiation oftermination so that current flows in one path after initiatingtermination of conduction for a short period and in which the means fordelaying provides a time that approximates the short period.
 3. Theinvention as defined in claim 1 in which the pulse-to-step control meanshas a pair of leads for each phase with the signal for a change ofenergization being a change in voltage levels on the leads, in which themeans for rendering the paths conductive or non-conductive includeslogic means in each path, there being one lead connected to one logicmeans and in which the logic means is responsive to only one voltagelevel on its lead to effect conduction and is responsive only to theother voltage level to effect non-conduction.
 4. The invention asdefined in claim 3 in which the logic means in each path includes a gatehaving a pair of inputs, in which the one lead is connected to the oneinput, in which the delaying means is operatively connected to the leadsand has an output connected to each of the other gate inputs and inwhich the delaying means output is altered for a time period after achange of voltage levels on the leads.
 5. The invention as defined inclaim 1 in which the source has two leads with a switching meansconnected between the source and the phase to provide two switchingmeans in each path and in which the means for rendering the pathsconductive or non-conductive includes means for simultaneouslyidentically altering the state of conduction of both switching means.